The Wi38 is a human embryo lung tissue in culture. These cells age with progressive cell division. The aging is accompanied by a cessation of division in an increasingly greater fraction of the total population until the culture cannot maintain itself and thus dies. Since cAMP effects cell division in this cell line, we postulated that this nucleotide system is affected by the aging process and may participate in it. The cAMP system is primarily a cellular messenger for extracellular stimuli. We are therefore further proposed that at least some of the changes in the cAMP system with aging occur at the plasma membrane level. We are presently continuing with the research generally as outlined in the original proposal. In addition to the cAMP system, we have also looked at the cyclic GMP system, substrate uptake, membrane fluidity and phospholipid content. We found no change in membrane fluidity or phospholipid content with passage, in the WI38 cultures. We found increases, however, in total cAMP and cGMP levels per cell, and a decrease in the number of transport sites in late passage WI38 cells.